It is very common to laminate two or more tissue plies in order to produce the final tissue product. Herewith a more flexible and softer tissue product is obtained with a laminated product as compared to if one single ply with a corresponding thickness and basis weight had been produced. The absorbent capacity and the bulk are moreover improved.
The lamination of two or more tissue plies is often made by means of gluing. A mechanical embossing of the plies is also often performed before they are glued together. It is further known to laminate two plies only by means of a mechanical embossing, at which a mechanical joining of the plies occurs in the embossing sites.
Through for example EP-A-796 727 it is known to first emboss two paper plies in a three dimensional structure with alternating raised and recessed portions, after which glue is applied to one of the plies and the two plies are joined in a press nip between two embossing rolls, so that the raised portions of the respective plies are glued to each other. A similar embossing procedure is shown in EP-A-738 588, according to which the glue also has a colouring effect.
In WO 95/08671 there is enclosed an example of so called nested embossing, in which the two individually embossed plies are combined and joined with the raised portions of one ply nesting into the recessed portions of the opposite ply.
Through U.S. Pat. No. 5,443,889 there is known a procedure for laminating two paper plies, which are fed over a pattern roll each, said pattern rolls having alternating raised and recessed portions and where glue is applied to one ply while this is led over the roll. The two paper plies are then glued together in a nip between the two pattern rolls, which are in register with each other so that a joining and compression of the paper plies occurs in a pattern corresponding to the protuberances of the pattern rolls.
A drawback that occurs in connection with embossing a paper web where this is compressed in spots, is that a considerable strength reduction occurs in the embossing sites, which affects the strength properties of the entire paper product. Strength reductions of up to 70% of an embossed paper as compared to a corresponding non-embossed paper are not unusual.
U.S. Pat. No. 3,672,950 discloses a method for producing a quilted or cushioned adhesively laminated tissue product in which glue is applied in a certain pattern to one tissue ply in a press nip between a first pattern roll and an impression roll. This ply is laminated to another ply in a press nip between the same impression roll and a second pattern roll having a pattern corresponding with that of the first pattern roll and driven in registry with the first pattern roll. The two plies are in different conditions of stress during the lamination process, so that a quilted or cushioned product is provided.
Laminating three or more tissue plies together by gluing can be made in different ways. According to one alternative embodiment an embossing unit comprising two embossing nips defined by an embossing roll and a marrying roll providing a nested configuration of the embossed and laminated web and further comprising a glue applicator, as disclosed in for example U.S. Pat. No. 3,867,225, is used, with the modification that two plies are fed together into at least one of the embossing nips. Using this technique for laminating three or more plies gives a rather stiff product since the glue has to penetrate completely through the middle layer(s) in order to glue all layers together. An insufficient penetration of the glue through the middle layer(s) will not provide a sufficient ply-bonding effect. The problem will be even more pronounced with four or more plies.
According to an alternative embodiment three or more plies may be joined together by mechanical ply-bonding by so called male-to-flat or male-to-male embossing. The embossing is normally made only along the edge portions of the tissue product, so called edge embossing, as is disclosed in for example U.S. Pat. No. 1,774,497, but may also be made over the entire surface of the product. EP-A-436 170 discloses a multiply tissue product comprising at two or more plies that are individually embossed and laminated together by edge embossing. It is difficult to achieve a sufficient ply-bonding with mechanical embossing only, especially when only the edge portions are embossed.
In Swedish patent application no. 0201088-2 there is disclosed that three or more plies are joined together by gluing in a plurality of discrete glue sites, wherein a first and second ply of said multi-ply web are glued together in a first glue pattern comprising a plurality of spaced discrete glue sites and a third ply is glued to the combined first and second plies in a second glue pattern comprising a plurality of spaced discrete glue sites which are offset with the glue sites of said first glue pattern.